Transparent single crystal scintillators are used to detect gamma rays, x-rays, cosmic rays, and other types of radiation, and to detect particles having energies of about 1 KeV and above. When radiation is incident on the scintillator, light pulses are generated by the scintillator that may be optically coupled to the photomultiplier tube of a scintillation detector to produce a voltage signal that is related to the number and amplitude of the light pulses received by the photomultiplier tube. Crystal scintillators are used in digital radiography, medical imaging, mineral and petroleum exploration, and other important applications.
A widely used scintillation detector employs the thallium-doped sodium iodide scintillator, Nal(TI); it has a very high light output (i.e., is a very bright scintillator) in response to radiation and is relatively inexpensive to produce. Scintillation detectors employing Nal(TI) are used in logging tools to aid in the location of petroleum deposits.
Inorganic metal oxides are another important group of materials used in scintillation detectors. These include bismuth germanium oxide Bi.sub.4 Ge.sub.3 O.sub.12 (BGO) and cerium-activated oxyorthosilicates, which include cerium-activated gadolinium oxyorthosilicate Gd.sub.(2-x) Ce.sub.x SiO.sub.5 (Ce:GSO), cerium-activated lutetium oxyorthosilicate Lu.sub.(2-x) Ce.sub.x SiO.sub.5 (Ce:LSO), and cerium-activated yttrium oxyorthosilicate Y.sub.(2-x) Ce.sub.x SiO.sub.5 (Ce:YSO). The data in The Table below, which is taken from the papers and patents that follow, summarizes relevant physical properties for Nal(TI), Ce:BGO, Ce:GSO, Ce:LSO, and Ce:YSO. The decay time in nanoseconds refers to the time it takes for a particular scintillator crystal to luminesce from the excited electronic state, which is the time required before the crystal can respond to additional radiation once it been exposed to sufficient radiation to produce an electronically excited state in the crystal. The reported range of decay times for several entries is likely a result of the difficulty in obtaining consistently uniform incorporation of cerium into the product crystal scintillator during crystal growth. The emission peak wavelength in nanometers refers to the wavelength maximum in the emission spectrum for the particular crystal scintillator.
TABLE 1 Property Nal(Tl) BGO Ce:GSO Ce:LSO Ce:YSO Density (g/cm.sup.3) 3.67 7.13 6.71 7.4 4.45 Relative light output 100 12 25 75 118 Decay time (ns) 230 300 60 40 40-70 Emission peak wavelength 410 480 430 420 420 (nm) Rugged No Yes No Yes Yes Hygroscopic Yes No No No No
U.S. Pat. No. 4,958,080 to C. L. Melcher entitled "Lutetium Orthosilicate Single Crystal Scintillator Detector," which issued on Sep. 18, 1990, describes Ce:LSO.
U.S. Pat. No. 5,025,151 to C. L. Melcher entitled "Lutetium Orthosilicate Single Crystal Scintillator Detector", which issued on Jun. 18, 1991, describes an apparatus that uses the Ce:LSO scintillator of the '080 patent to investigate subsurface earth formations.
"Czochralski Growth of Rare Earth Oxyorthosilicate Single Crystals" by C. L. Melcher et al. was published in J. Crys. Growth, vol. 128, p. 1001-1005, (1993) and describes using the Czochralski crystal growing method to prepare single crystals of Ce:GSO, Ce:LSO, and Ce:YSO.
U.S. Pat. No. 5,660,627 to R. A. Manente et al. entitled "Method of Growing Lutetium Oxyorthosilicate Crystals," which issued on Aug. 26, 1997, describes an improved Czochralski crystal growth method for growing an LSO crystal that displays substantially uniform scintillation behavior throughout the crystal. Also described is a scintillation detector used with the crystal. "Physical Processes in Inorganic Scintillators" by P. A. Rodnyi, p. 50, CRC Press, New York, N.Y. (1997), includes data relating to Ce:YSO.
Ideally, a crystal scintillator is inexpensive to produce, has a fast decay time, and is dense, bright, and is a rugged crystal. As The Table clearly demonstrates, the decision to use a particular scintillator involves compromises between the various physical properties. Although Nal(TI) is a very bright crystal scintillator, it is not dense so that much of the radiation incident on the crystal is not absorbed by the crystal. Due to its hygroscopic nature, Nal(TI) must be protected from moisture and because it is not rugged, it should not be used in applications where it is subject to fracture. Finally, Nal(TI) has the relatively long lumiscence decay time of over 400 ns.
BGO is almost twice as dense as Nal(TI) and is a rugged and non-hydroscopic crystal. However, BGO is not as bright a crystal as Nal(TI) and has an even longer decay time. Ce:GSO is also a dense crystal scintillator and is a brighter crystal than BGO. However, Ce:GSO is not a rugged crystal.
Ce:YSO is a bright, rugged, non-hygroscopic crystal. Importantly, the starting yttrium oxide Y.sub.2 O.sub.3 which is used to grow Ce:YSO is relatively inexpensive, about $20/kg for 99.99% pure Y.sub.2 O.sub.3. Ce:YSO has a melting temperature of about 2000.degree. C., which is about 150 degrees lower than the melting temperature for Ce:LSO, making fabrication of Ce:YSO easier and less energy demanding than that for Ce:LSO. Unfortunately, Ce:YSO is not a very dense crystal, and decay times as long as 70 ns have been reported for this material.
Of the scintillators listed in The Table, Ce:LSO has the most desirable physical properties; it is a bright, dense, rugged, non-hygroscopic scintillator, and has a short decay time. However, Ce:LSO is extremely expensive, about $2,000/kg for 99.99% pure material. In addition, the processing temperature for growing Ce:LSO is very high; Lu.sub.2 O.sub.3 and LSO melt at temperatures of about 2310.degree. C. and 2150.degree. C., which adds to the difficulty of growing crystals of growing Ce:LSO.
Efforts to provide oxyorthosilicate scintillators with a broader range of properties have led to the production of cerium-activated single crystal scintillators having compositions that include a variety of lanthanide elements in combination with Gd, Lu, and Y. Examples of these are described in the papers and patents that follow. "Czochralski Growth of Rare-Earth Orthosilicates (Ln.sub.2 SiO.sub.5)" by C. D. Brandle was published in J. Crys. Growth, vol 79, p. 308-315, (1986) and provides an evaluation of the Czochralski method for growing GSO, YSO, and a variety of orthosilicates containing either Gd or Y doped with a lanthanide series element. The reported combinations with Y were YSO doped with Ce, Pr, Nd, Sm, Gd, Tb, Er, Tm, and Yb. The reported combinations with Gd were GSO doped with Ce and Tb. "Czochralski Growth and Characterization of (Lu.sub.1-x Gd.sub.x).sub.2 SiO.sub.5 " by G. B. Loutts et al. entitled was published in J. Crys. Growth, vol. 174, p. 331-336, (1997), and describes single crystal oxyorthosilicate scintillators having both Lu and Gd.
U.S. Pat. No. 4,647,781 to K. Takagi et al. entitled "Gamma Ray Detector," which issued on Mar. 3, 1987, describes a cerium-activated oxyorthosilicate scintillator having both Gd and Y and/or La. These scintillators have the general formula Gd.sub.2(1-x-y) Ln.sub.2x Ce.sub.2y SiO.sub.5 where Ln is yttrium and/or lanthanum, where 0.ltoreq.x.ltoreq.0.5, and 1x10.sup.-3.ltoreq.y.ltoreq.0.1.
U.S. Pat. No. 5,264,154 to S. Akiyama et al. entitled "Single Crystal Scintillator," which issued on Nov. 23, 1993, describes a single crystal scintillator and apparatus for prospecting underground strata using the scintillator. The single crystal scintillator is a cerium-doped oxyorthosilicate having the general formula Gd.sub.2-(x+y) Ln.sub.x Ce.sub.y SiO.sub.5 wherein Ln is Sc, Tb, Lu, Dy, Ho, Er, Tm, or Yb, 0.03.ltoreq.x.ltoreq.1.9, and 0.001.ltoreq.y.ltoreq.0.2.
Clearly, it is desirable to provide an affordable crystal scintillator having the most desirable properties for a particular application.
Therefore, an object of this invention is to provide an oxyorthosilicate crystal scintillator that can be used to detect gamma rays, x-rays, and the like.
Another object of the invention is to provide a crystal scintillator having excellent physical properties at a reasonable cost. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.